1. Field of the Invention
The present invention generally relates to an electrolytic process and electrolytic cell for electrolysis of an aqueous alkali metal halide solution, especially an aqueous alkali metal chloride solution. More particularly, it relates to a process and apparatus for mainly obtaining a high purity caustic alkali more effectively with low cell voltage using a horizontal type electrolytic cell providing a cation exchange membrane as an electrolytic separator.
2. Description of Prior Art
The most typical horizontal electrolytic cell is a mercury electrolytic cell; however, these are destined to be shut down in the near future since mercury, used as a cathode, contaminates the environment. When such a mercury cathode electrolytic cell is desired to be converted into a separator electrolytic cell employing no mercury with a reduced cost, the separator electrolytic cell should be of a horizontal type. In view of the situation, it is a significant matter the industry is now encountering to develop a process for producing a high purity product, not inferior to a product by the mercury process, with a high current efficiency using such horizontal type separator electrolytic cells.
A process for remodeling a mercury cell to a horizontal type separator cell is revealed in the U.S. Pat. No. 3,923,614. In the process, however, a porous membrane (diaphragm) is used to serve as a separator, having great water permeability and accordingly anolyte solution passes through the separator hydraulically to thus mingle in, for example, caustic alkali produced in the cathode compartment, thereby resulting in decreased purity.
On the other hand, a cation exchange membrane called a non-porous membrane permits no passage of anolyte solution or catholyte liquor hydraulically, allowing only water molecules coordination-boned to alkali metal ions transported electrically to pass, hence a high purity caustic alkali being obtained. To the contrary, a small quantity of water transported evaporates to cause electric conduction failure between a membrane and a cathode, in the long run to terminate electrolytic reaction.
The U.S. Pat. No. 3,901,774 proposes processes to solve these problems; one is a process for placing a liquid maintaining material between a cation exchange membrane and a cathode and another is a process for carrying out the electrolysis while supplying to a cathode an aqueous caustic alkali liquor in mist or spray.
Notwithstanding, the former process not only involves the problems including troubles for interposing the liquid maintaining material and the durability thereof, but increases cell voltage because the distance between electrodes is expanded by the liquid maintaining material located between the cation exchange membrane and the cathode, besides an increase in electric resistance of the liquid maintaining material per se. Hence it can not be an advantageous process. Moreover the latter process has some difficulties in practice on an industrial scale since the uniform supply of liquid is difficult when applied to a large-scale electrolytic cell such as employed commercially.
In an attempt to eliminate the foregoing defects attendant on the conventional processes, a process and apparatus therefor has been proposed by Ser. No. 434,737 (EPC Appln. No. 82109528). This proposal involves a process for enfolding hydrogen gas generated on a cathode in a catholyte liquor stream to thereby remove hydrogen gas from a cathode compartment, and electrolytic cell which is characterized by an upper anode compartment and a lower cathode compartment partitioned by a cation exchange membrane positioned substantially horizontal, said anode compartment having therein substantially horizontal anodes and being surrounded by the top cover, side walls positioned so as to enclose the anodes and the upper side of the membrane, and being provided with an inlet and an outlet of anolyte solution and an outlet of anode gas, said cathode compartment being surrounded by a cathode plate having gas-liquid impermeability, side walls so as to enclose the cathode plate and the underside of the membrane, and being provided with an inlet of catholyte liquor and an outlet of a mixed stream of the cathode gas and the catholyte liquor.
In carrying out the electrolysis using such type construction cell, it is, first of all, an exceedingly essential point to cause cathode gas generated in the cathode compartment to be rapidly enfolded in the catholyte liquor stream and to prevent gas-liquid separation in the cathode compartment. In this point, the initial linear velocity of catholyte liquor supply to the cathode compartment has a close bearing on the residence of gas and cell voltage and high purity caustic alkali can be obtained with low cell voltage without residence of gas by controlling the initial linear velocity in the cathode compartment to about 8 cm/sec or more. Even when, however, the initial linear velocity is maintained at 8 cm/sec or more, fine gas bubbles aggregate with an increase of gas to thus cause gas-liquid separation within the cathode compartment, and gas layer separated from liquid covers the underside of the membrane to result in an increase in cell voltage. Moreover, the gas prevents long-term and stable operation because of vibration of the membrane caused by the intermittent withdrawal of gas separated and damages the membrane in the end.
Secondly, in effecting the electrolysis while circulating the catholyte liquor along the longitudinal way of the cathode plate, it has been made clear the following problems occur. That is: (a) With the linear velocity of 50 cm/sec at an inlet of the catholyte liquor, pressure difference (.DELTA.p) between the cathode compartment and the anode compartment at the neighborhood of the catholyte liquor inlet becomes about 0.3 Kg/cm.sup.2 and the load ammounting to several tens tons is imposed on the whole cathode compartment. As a result, the cathode plate, a DSE (dimensionally stable electrode) and a cell cover are not only deformed, the distance between electrodes being expanded to thus raise cell voltage, but the membrane is damaged. On the other hand, to prevent such deformation of the cathode plate, the DSE and the cell cover, reinforcement is required, thereby leading to a complicated structure as well as increased cost; (b) G/(L+G) (content of cathode gas contained in unit volume of a mixture of cathode gas and the catholyte liquor) of the catholyte liquor increases to thus raise electric resistance of the mixed stream consisting of the catholyte liquor and the cathode gas, in consequence, current distribution takes place in a longitudinal direction of the cathode plate. For instance, at current density of 20 A/dm.sup.2 .DELTA.CV (difference in cell voltage between at the inlet and the outlet of the catholyte liquor) reaches approximately 40 mV; (c) Fine gas bubbles aggregate in the mixed stream to thus cause gas-liquid separation which permits a pulsating flow to occur. For this reason, .DELTA.p varies to thus vibrate the membrane and when the distance between electrodes is small, contact and separation of the membrane and the electrodes is repeated, thereby resulting in damage of the membrane.
Thirdly, because of pressure loss generated from the catholyte liquor inlet to the catholyte liquor outlet, the pressure of the catholyte liquor is high in the vicinity of the catholyte liquor inlet and becomes close to zero in the vicinity of the catholyte liquor outlet. Therefore, in the vicinity of the catholyte liquor outlet a slight change in pressure between the anode and cathode compartments permits the cation exchange membrane to vibrate and occasionally injures the membrane in a form of a crack, wear, pin-holes and the like during operation for a long period of time. A change in pressure imposed on the cation exchange membrane takes place, for example, when the mixed stream of the cathode gas and the catholyte liquor partly causes gas-liquid separation in the neighborhood of the catholyte liquor outlet to thus permit the residence of gas, whereby pulsating flow is partly brought about.
Fourthly, when the catholyte liquor is introduced and removed parallel to a circulating direction thereof, the inlet (19) and the outlet (20) are usually positioned between the membrane (3) and the cathode plate (16), namely, to side walls of the cathode compartment, as illustrated by FIG. 6. Accordingly, even though the membrane-cathode plate distance is desired to be smaller than the space of the inlet or outlet, various difficulties arise and when practiced daringly, the structure is complicated and equipment cost is increased.
Furthermore, it has been found by the inventors that in a liquid-contacting and electric current-nonpassing portion of the membrane (non-electrolysing portion), NaOH migrates, for instance, in electrolysis of an aqueous NaCl solution, through the membrane into the anolyte solution to thus reduce solubility of NaCl in the anolyte solution, NaCl being therefore deposited on the membrane. The liquid-contacting and current-nonpassing portion means a portion in contact with the anolyte solution and/or the catholyte liquor and substantially not opposing the anode plate and the cathode plate, where substantially no electrolysis takes place. This phenomenon, as shown by FIG. 7, is apt to occur at a portion where the membrane (3) is sandwitched between a flange (5a) and a side wall (17) of the cathode compartment. NaCl deposited on the membrane not only presses down the membrane to thus change the electrodes-membrane distance, but also induces the membrane to vibrate and sometimes damages it owing to collision of the membrane and the electrodes. Moreover the phenomenon varies the flow rate of the catholyte liquor in the cathode compartment. When the foregoing phenomenon takes place in the neighborhood of the catholyte liquor inlet and the mixed stream outlet, those inlet and outlet are choked and pressure loss resulting from the flow of the catholyte liquor is increased. As a result, long-term and stable operation is impossible.